In a capacitive touch-sensitive display device, an image may be displayed while simultaneously detecting touch input from a user's digit or other input device. The capacitive touch-sensitive display device may include a touch surface, a display stack such as a liquid crystal display (LCD) stack, and a matrix of column and row electrodes positioned therebetween and configured to detect touch input based on changes in capacitance, for example the change in capacitance between columns and rows, or the change in capacitance between a row or column and ground. Such changes in capacitance may be used to determine a column-row pair closest to the touch input, and to determine the degree to which the touch input is off-center relative to the column-row pair. In this way, touch input may be detected and interpreted at high resolutions to control aspects of a computing device.
In one prior capacitive touch sensor design, an opaque metallic conductor is used for the capacitive touch sensor, and the column and row electrodes are oriented substantially vertically and horizontally relative to the LCD, between the display stack and adjacent the touch surface. However, with this design, the conductive elements visually occlude portions of the display stack, causing the user to perceive the presence of the touch sensor due to the formation of various artifacts created by the electrode-display occlusion. The perceptibility of these artifacts changes with the viewing angle of the viewer, but is particularly visible in such a design because the columns and rows are oriented vertically and horizontally, parallel to the underlying vertical columns and horizontal rows of the underlying pixels in the LCD.
In another prior capacitive touch sensor design, a capacitive touch sensor may be comprised of a transparent conductive oxide (TCO) such as indium tin oxide (ITO), to reduce visual perceptibility. When used in large format devices, however, TCO touch sensors have an electrical resistance that, in combination with the capacitances under test and certain other stray capacitances, may result in an RC time constant slow enough to limit the achievable excitation frequency of the touch sensor, and thus limit the achievable frame rate for a desired SNR. As a result, TCO is generally limited to application in displays with less than a roughly 30 inch diagonal dimension.
As discussed in more detail below, challenges exist for minimizing the visual perceptibility of capacitive touch sensors for large format capacitive touch-sensitive display devices. These challenges have generally slowed the development and adoption of such devices in the marketplace.